Mayor Sadiq Khan’s proposals to take over surface rail services in the capital have been branded “unfunded” and “uncosted” by transport secretary Chris Grayling.

Transport for London already oversees the London Overground and will be responsible for Crossrail when passenger services get underway. The Mayor’s transport agency also manages the London Underground and DLR.

Politicians from all major parties in the capital support devolving additional services, a move they say will improve the quality of service and potentially lower fares for passengers.

However in December Mr Grayling abandoned a previously agreed deal for the Department for Transport and TfL to work towards the devolution of rail services not yet overseen by the Mayor. Instead of handing over full control, the DfT now says TfL will merely be invited to help design new franchises, starting with the forthcoming Southeastern contract.

Days after that decision was announced, it emerged that Mr Grayling had previously written to former Mayor Boris Johnson expressing a desire to keep services “out of the clutches” of Labour.

News of Grayling’s letter, which was written three years ago in his capacity as MP for Epsom and Ewell, prompted accusations that he had allowed party politics to influence his thinking.

Former Conservative London Assembly member and current Bromley & Chislehurst MP publicly criticised his colleague and called for his resignation.

Appearing before MPs on Thursday, Mr Grayling was challenged by Greenwich MP Matthew Pennycook who said Londoners and other rail users would be “disgusted” that ministers were putting politics above passenger needs.

However the minister denied the charge and said his decision to drop rail devolution had been taken “after the mayor’s business plan was analysed” across government and after discussions with neighbouring local authorities.

Grayling said Mr Khan’s plans “offered no extra capacity and a whole lot of unfunded, uncosted promises” which hinged on “a very substantial top down reorganisation.”

He added that inviting TfL to design future contracts mirrored the approach being taken across England.